[Haskell-cafe] Does GHC compare pointers when eval'ing (==)

Michael Snoyman michael at snoyman.com
Wed Aug 20 08:35:05 UTC 2014

On Wed, Aug 20, 2014 at 11:28 AM, Johan Tibell <johan.tibell at gmail.com>

> On Wed, Aug 20, 2014 at 10:23 AM, Erik Hesselink <hesselink at gmail.com>
> wrote:
>> As I understood, the question was if GHC would first compare pointers,
>> and only call the Eq instance if the pointers are not equal. I guess
>> this would be safe, but I don't think GHC does such a thing.
> I think the reason it isn't done is that it's not always an optimization.
> We do it manually in e.g. bytestring.
There are two cases I can think of where it would also change the semantics
of the code:

1. An Eq instance that doesn't obey the reflective property (not

data BadEq = BadEq
instance Eq BadEq where
    BadEq == BadEq = False

2. Eq instances intended to avoid timing attacks, by always comparing the
entire data structure.

newtype SlowEq a = SlowEq [a]
instance Eq a => Eq (SlowEq a) where
    SlowEq x == SlowEq y = slowAnd $ length x == length y : zipWith (==) x y

slowAnd =
    loop True
    loop !x [] = x
    loop !x (!y:ys) = loop (x && y) ys

(Note: not actually tested.) It's difficult for me to imagine a case though
where a timing attack could result on two data structures sharing the same
pointer; usually we'd be talking about comparing two ByteStrings or Texts
that come from very different sources.

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